This invention relates to a method of field-flow fractionation (FFF), and more particularly to the use of high field gradients to establish particle residence at the wall of a field-flow fractionation system.
FFF, disclosed previously in U.S. Pat. No. 3,449,938, is the descriptive term referring to a broad field of technology developed primarily for separation and characterization of macromolecules and particles. Generally, FFF has demonstrated a capability to deal with extreme ranges of mass, including particles sizes varying from a molecular weight of 600 to particles of almost 1 micrometer in diameter.
As explained in the referenced patent and also in a previous patent application of the present inventor (U.S. patent application Ser. No. 810,835, now U.S. Pat. No. 4,147,621), FFF involves the differentiation and segregation of particles along a flow channel under the influence of a force field applied across the flow channel. The effect of this field, which is usually applied perpendicular to the flow channel, is to force particles of different sizes into equilibrium layers of different effective thickness against a channel wall which operates as a restraining wall with respect to the particles. The thickness of the layers is determined primarily by (1) the interplay between the field-induced forces which tend to compact particles agains the restraining wall and (2) Brownian motion which tends to disperse the particles away from the wall.
Generally, the operation of FFF causes the largest particles to form the most highly compressed layer, located in the channel flow region immediately adjacent the restraining wall. Conceptually, this channel flow is illustrated in FIG. 1 which shows the differential velocities (V.sub.1, V.sub.2, V.sub.3, V.sub.4, etc.) of the flow stream across the width of the channel w. It will be noted that the highest velocity (V.sub.4) is toward the central region of the channel, with the lowest velocity (V.sub.1) occurring at the opposing walls. A field gradient of strength G disposed perpendicular to the flow channel tends to drive the particles toward the restraining wall of the channel as shown.
In all cases of FFF the segregation of particles along the length of the channel occurs because of the differential displacement of particles having different effective layer thickness. In the conventional practice of FFF, the field strength G applied is sufficiently low to permit Brownian motion to displace small particles toward the high velocity central region of the channel, while the larger particles which interact more strongly with the field are retained in layers closer to the wall. It is the differential location of the particles by size along the velocity profile of the channel which causes the fractionation of the particles into particle size groups.